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Magnon Resilience in Diluted Antiferromagnets
Y-doped TbSb · Crystal-field hybridization · Inelastic neutron scattering
TbSb is a rare-earth antiferromagnet with a rock-salt crystal structure that orders below TN ≈ 15 K. The Tb³⁺ ions carry a large magnetic moment and experience a strong cubic crystal-field from the surrounding Sb cage, which splits the J = 6 ground multiplet into a well-defined level scheme. When non-magnetic yttrium is substituted for magnetic terbium, the magnetic sublattice is progressively diluted. Standard percolation theory predicts that once the concentration of magnetic sites drops below the percolation threshold, long-range order and coherent spin waves should break down.
In practice, the spin waves in Y-doped TbSb are far more robust than this picture suggests. Using inelastic neutron scattering across a series of (Tb1−xYx)Sb samples, I am tracking the magnon spectrum as a function of yttrium content and finding that magnon branches remain well-defined at doping levels where simple models predict their collapse. Crystal-field hybridization between single-ion and collective degrees of freedom appears to be the key mechanism that thermally stabilizes the magnon modes against dilution.
TbSb is an ideal platform for studying diluted magnetism precisely because its single-ion physics is well characterized. The crystal-field parameters for Tb³⁺ in the cubic Sb environment are known from earlier optical and neutron spectroscopy, the magnon dispersion in the parent compound is cleanly resolved, and Y³⁺ substitutes for Tb³⁺ without changing the lattice structure or introducing charge disorder. This means that any deviation from simple percolation behavior can be traced directly to the crystal-field coupling rather than to competing structural or electronic effects.
This is the key advantage of TbSb over more complex diluted magnets, where multiple mechanisms are simultaneously active and difficult to disentangle from scattering data alone. TbSb allows a clean, controlled test of whether and how single-ion anisotropy protects collective magnetism in a disordered system.
In rare-earth magnets, the crystal-field splits the ground-state multiplet into a set of levels. When the separation between the ground state and excited crystal-field levels is comparable to the magnon bandwidth, the two types of excitation hybridize, producing mixed magnon–crystal-field (magnon-CEF) modes. This hybridization is a purely quantum mechanical effect with no classical analogue and can substantially alter the magnon dispersion, lifetime, and gap.
In the diluted system, the local crystal-field environment of each Tb³⁺ site depends on the surrounding Y/Tb configuration, which broadens the distribution of crystal-field splittings across the sample. Our measurements aim to determine how this distributed coupling feeds back into the collective magnon spectrum — specifically whether the effective crystal-field hybridization pins or broadens the magnon branches, and how this changes with doping level and temperature.
Understanding when and why collective spin excitations survive magnetic disorder is a foundational question in quantum magnetism. Magnon coherence length governs thermal conductivity in magnetic insulators, sets the range of spin-wave propagation in magnonic devices, and determines the fidelity of quantum correlations in candidate quantum materials. Establishing the conditions under which dilution-induced disorder destroys or preserves magnon coherence — and identifying the microscopic mechanism responsible — provides design principles for disorder-tolerant magnetic systems across a wide range of potential applications.